It is a common practice in the fabrication of integrated circuits to make use of an interconnecting layer of aluminum (which has high conductivity while also being readily deposited and etched) for connecting the different elements of the integrated circuit to each other. This layer usually rests on an insulating layer of relatively substantial thickness (of the order of one micron or more) which is in turn located above the subjacent elements of the integrated circuit. The insulating layer is opened prior to deposition of the aluminum in order to expose the surfaces with which it is desired to establish a metallic interconnection. These surfaces can be monocrystalline silicon surfaces (transistor sources and drains, collectors, bases and emitters) or polycrystalline silicon surfaces (field-effect transistor gates) or even metallic surfaces of another interconnecting layer.
The substantial thickness of the insulating layer arises from the need to reduce stray capacitances between the interconnecting conductors and the circuit elements above which said conductors pass.
Unfortunately, this thickness makes it difficult to obtain contacts having surfaces of very small area and located at very small intervals between the interconnecting layer and the circuit elements.
In fact, should it be desired to have narrow contact surfaces located very close together, it is found necessary to carry out (vertical) directional etching of the contact openings in the insulating layer. However, if special precautions are not taken, this directional etching produces openings having sharp edges which are liable to result in fractures of the aluminum at the time of deposition by vacuum evaporation. The precautions which may be taken consist in particular of deposition of the insulating layer in two steps and etching of the layer also in two steps. However, these precautions produce an increase in the pitch between conductors (pitch=width+spacing) at the level of the contacts.